Implantable cardiac stimulation devices are well known in the art. Such devices may include, for example, implantable cardiac pacemakers and defibrillators. The devices are generally implanted in a pectoral region of the chest beneath the skin of a patient within what is known as a subcutaneous pocket. The implantable devices generally function in association with one or more electrode carrying leads which are implanted within the heart. The electrodes are usually positioned within the right side of the heart, either within the right ventricle or right atrium, or both, for making electrical contact with their respective heart chamber. Conductors within the leads and a proximal connector carried by the leads couple the electrodes to the device to enable the device to sense cardiac electrical activity and deliver the desired therapy.
Traditionally, therapy delivery had been limited to the venous, or right side of the heart. The reason for this is that implanted electrodes can cause blood clot formation in some patients. For example, a blood clot released arterially from the left heart, as for example from the left ventricle, could pass directly to the brain potentially resulting in a paralyzing or fatal stroke. However, a blood clot released from the right heart, as from the right ventricle, would pass into the lungs where the filtering action of the lungs would prevent a fatal or debilitating embolism in the brain.
Recently, new lead structures and methods have been proposed and even practiced for delivering cardiac rhythm management therapy to the left heart. These lead structures and methods avoid direct electrode placement within the left atrium and left ventricle of the heart by lead implantation within the coronary sinus region of the heart. As used herein, the phrase “coronary sinus region” refers to the venous vasculature of the left ventricle, including any portions of the coronary sinus, great cardiac vein, left marginal vein, left posterior ventricular vein, middle cardiac vein, and/or small cardiac vein or any other cardiac vein accessible by the coronary sinus.
Studies have shown that patients suffering from congestive heart failure (CHF) may exhibit marked improvement with left heart pacing. This is attributed to improved cardiac output and decreasing mitral valve regurgitation. These improvements decrease mortality rates and provide the patients with a higher quality of life. Left heart pacing can also improve the condition of patients suffering from left ventricular dysfunction. The preferred pacing configuration for these patients is dual chamber pacing wherein either the right or left atrium is paced along with the left ventricular.
Left heart pacing via the cardiac sinus region can be difficult as stimulation sites vary from patient to patient due to normal anatomical variations. Also, the location and interior size of the vessels and optimal location of the stimulation sites are complicated by an increase in heart size associated with long term CHF. In addition, many patients suffering from CHF have also undergone myocardial infarct(s), bypass surgery, and/or other types of invasive cardiac surgery. Each of these conditions change the position of the vessels within the heart and the position of the stimulation sites.
It is therefore generally thought that left heart pacing requires two separate leads, one lead for placing at least one electrode in either the right atrium or within the coronary sinus adjacent the left atrium and another lead for placing at least one other electrode within the coronary sinus adjacent the left ventricle. However, implanting two such separate leads can be difficult, especially if both leads are to be implanted in the coronary sinus. For example, the implant can be time consuming. Further, there is an inherent risk of interaction between the leads. Moreover, there is an increased risk that one of the leads will become dislodged.